The present invention relates in general to inflators used in airbag restraint systems. In particular, the present invention relates to an improved inflator having heated stored gas, with central discharge of the gas and soft onset of cushion inflation.
Airbag restraint systems typically include a controller for determining when a collision occurs, an inflator which receives a signal from the controller to produce a quantity of inflation gas, and a cushion which receives, and is inflated by, the inflation gas. The vehicle occupant compresses the inflated cushion to reduce the collision force and reduce or eliminate injury. Various types of inflators are known, and are generally grouped according to their operating principles.
One preferred inflator type employs a quantity of stored gas and a heater for the gas. It is well known that heating a gas will increase its expanded volume. As such, the heater will heat the stored gas, increasing its expanded volume. This permits a smaller quantity of gas to be stored, reducing the size of the pressure vessel holding the stored gas. Additionally, the heater itself may produce gas (such as during a combustion or reaction) which will combine with the stored gas to inflate the cushion, further reducing the quantity of stored gas required. In addition to increasing the expanded volume, the heater also typically raises the pressure of the stored gas during inflator functioning. This can have a negative effect, in that it increases the structural requirements (and therefore weight) of the pressure vessel. Many inflators of this type rely upon the pressure increase in the stored gas, due to operation of the heater, to rupture a diaphragm, which starts the flow of inflation gas from the inflator. This creates a high initial rate of gas flow into the cushion, which is not desirable for inflation. Specifically, a high initial gas flow can rapidly expand the cushion with considerable force, which may cause injury to an occupant who is out of the normal seating position and in the path of the deploying cushion. It is therefore desirable to provide an inflator which initially releases unheated stored gas (low initial rate of gas flow), followed by heating of the remaining stored gas to still provide for the increased expanded volume benefit of the heater. As such, the delayed operation of the heater may be timed to provide an initially lower gas flow rate, which increases smoothly. This initial low gas flow rate which increases smoothly is often referred to as a "soft onset". As such, these inflators may provide a very advantageous inflation of the cushion.
A further problem associated with these inflators is the flow of the gas influencing the direction in which the cushion deploys during inflation. The cushion must move quickly to the desired position in front of the vehicle occupant in order to provide protection. Many inflators of this type, however, cause the gas to flow into the cushion in a manner which skews deployment, moving the cushion in a direction other than directly toward the vehicle occupant.
This is due to the placement of the gas outlet vents on the end of the inflator. The most common shape for the stored gas container is an elongated tank, similar to tanks for storing oxygen, air, or other compressed gas. To ensure the tank has sufficient strength to withstand the pressure of the stored gas, the outlet is located at one longitudinal end of the tank, again similar to other stored gas tanks. The heater is typically mounted within the tank. While the inflator outlet is located at this one end, the open mouth of the cushion is typically mounted beside the inflator, and extends along the full length of the inflator. This provides a sufficiently large opening to permit rapid inflation of the cushion, while keeping the airbag system to a reasonably small size.
When activated, the gas exits the inflator very rapidly, and enters the mouth of the cushion. The placement of the outlet at one end of the inflator (and thus at one side of the cushion mouth) causes a large flow of gas to impinge upon the inflating cushion along that one side. This large flow often forces the cushion to deploy toward that side. While a diffuser may be placed between the inflator and cushion to assist in a more even distribution of the flowing gas, this problem continues to cause concern.